The term "multi-core optical fiber" refers to an optical fiber comprising N mutually parallel optical cores (where N is not less than 4) embedded in common optical cladding, the majority of the light rays conveyed by the multi-core fiber being guided along the N cores thereof. Conventionally, each of the N cores of the multi-core fiber has a diameter of a few microns (in general in the range 7 .mu.m to 10 .mu.m) and is disposed on a circle of radius approximately equal to 40 .mu.m inside the optical cladding, which optical cladding has an outside diameter of 125 .mu.m.
A "single-core fiber" is a conventional optical fiber comprising one optical core surrounded coaxially by optical cladding, the majority of the light rays conveyed by the single-core fiber being guided along the core thereof. The diameter of the core of such a fiber is also in the approximate range 7 .mu.m to 10 .mu.m, the outside diameter of the optical cladding being 125 .mu.m.
Naturally, both single-core fibers and multi-core fibers are surrounded by at least one protective layer of a suitable resin.
In order to couple a multi-core fiber of the above-mentioned type to a plurality of single-core fibers, in particular for branch connections, or end-of-line connections, etc., the most-commonly envisaged solution consists in performing coupling at an end section of the multi-core fiber by butt coupling one or more cores of the multi-core fiber to the cores of the single-core fibers. For example, such a solution for coupling a fiber having two cores to two single-core fibers by means of an intermediate element is described in an article entitled "Single-core to twin-core fiber connector" published in "Electronics Letters" dated Aug. 15, 1991, vol. 27, No. 17.
That solution is not satisfactory for coupling a multi-core fiber that includes at least four cores. Given the overall size of single-core fibers (the outside diameter of each single-core fiber is equal to that of the multi-core fiber), such butt coupling is in practice complex to perform without using an intermediate device (coupling tube, plane waveguide, etc.). But the accuracy required in aligning the intermediate device with the fibers in order to obtain the desired coupling, and the complexity of machining the intermediate device make that solution complex and costly.
Moreover, that solution does not make it possible to perform mere branch connections, i.e. to couple only some of the cores of the multi-core fiber: since the coupling is performed end-to-end, the optical transmission line must be cut, and putting back together its non-coupled cores is difficult.
In order to mitigate those drawbacks, an alternative solution could be applied that is known for coupling together two single-core optical fibers. That solution consists in performing lateral evanescent-wave coupling, and it is described, in particular, in an article entitled "Fabrication of an access coupler with single-strand multimode fibre waveguides" published in "Applied Optics" dated Nov. 1976, vol. 15, No. 11. in that solution the two single-core fibers to be coupled together are secured together over an interaction length that is a function of the desired coupling.
That technique cannot be applied to multi-core fibers since, for reasons of overall size and available space, it is impossible to dispose all the individual fibers at the same place around the multi-core fiber.